Recently high-performance polymers are demanded by the market. Although liquid-crystal polymers having specific properties such as optical anisotropy, satisfy the demands of the market under these circumstances, they sometimes have problems in that the melting points are excessively high due to their molecular chain structures and they tend to be economically expensive, since special monomers are to be used for producing them.
In particular, wholly aromatic polyesters now available on the market are mainly formed from hydroxy acids, particularly p-hydroxybenzoic acid, However, since the melting point of a homopolymer of p-hydroxybenzoic acid is higher than its decomposition point, the melting point must be lowered by copolymerizing it with various components.
A wholly aromatic polyester produced by using 1,4-phenylenedicarboxylic acid, 1,4-dihydroxybenzene, 4,4'-dihydroxybiphenyl, etc., as the comonomer components has a melting point off as high as 350.degree. C. or above which is too high to allow melt-processing with an ordinary apparatus. On tile contrary, although a wholly aromatic polyester produced by using 2,6-naphthalenedicarboxylic acid has a melting point of as low as around 300.degree. C. and excellent properties, a special comonomer component is necessitated for producing it.
As wholly aromatic polyesters produced without using p-hydroxybenzoic acid, ones produced by using a dihydroxyarylene or dicarboxyarylene as the monomer are described in Japanese Patent Publication-A No. 26681/1989. However, these polymers are economically disadvantageous, since a special monomer having a substituent in the aromatic ring is necessitated for lowering the melting point.